where \(\mathbf {H}_{k} \in \mathbb {C}^{N_{r, k} \times N_{t}}\)  denotes
the massive MIMO channel matrix between the BS and user \(k\) . As
discussed in [1]} and [2]}, this matrix
is conveniently described by a geometry clustered channel model with
\(N_{C}\)  clusters and \(N_{R}\)  rays in each cluster due to the sparse
scattering property of massive MIMO channels. Considering a system with a
half-wave spaced uniform linear array at both the transmitter and the
receiver, we have without loss of generality
